1. Field of the Invention
The present invention relates to an electron tube type imaging device.
2. Related Background Art
An imaging device is classified into a solid-state imaging device and an electron tube type imaging device. The solid-state imaging device has an essential disadvantage in that a sensitivity is insufficient when a size of picture element is reduced to improve a resolution power. In order to solve this problem, a laminated solid-state imaging device comprising a charge transferring device such as CCD laminated by a photo-conductive layer, in which an aperture ratio (a ratio of a light detection area to a surface area of the device) is increased has been developed, but an essential improvement of the sensitivity cannot be expected from the change of the surface area ratio of the device.
On the other hand, the electron tube type imaging device has a high sensitivity but it lacks stability and reliability. This will be described below in detail.
In the electron tube type imaging device, an incident light is first photo-electrically converted and a resulting electronic image is converted to a fluorescent image so that an image is picked up optically, or the image is picked up as an electronic image. In such an imaging device, a solid-state imaging device such as one shown in FIG. 5 may be used. As shown, an n-type layer 12 which serves as an electron charge accumulation area and an n-type layer 13 which serves as an electron charge transfer area are formed on a p-type substrate 11, and a top side thereof is covered with an insulation layer 14 such as SiO.sub.2. A transfer gate 15 and a shielding film 16 are formed in the electron charge transfer area of the insulation layer 14. In such an imaging device, when a signal (electrons) is applied as shown by an arrow, electron-hole pairs in the amount corresponding to an input energy are generated in the vicinity of a p-n junction of the p-type substrate 11 and the n-type layer 12, and the multiplied electrons (signal electrons) are transferred and read by the electron charge transfer area comprising the n-type layer 13 and the transfer gate 15.
This type of solid-state imaging device in which the electrons are directed to the front surface of the substrate may be called a front surface input type device while a device in which the electrons are directed to a back surface of the substrate may be called a back surface input type device. In the back surface input type device, the p-type substrate 11 is thinly machined as shown by a broken line L in FIG. 5. In this case, again, the input electrons are multiplied in the vicinity of the p-n junction of the p-type substrate 11 and the n-type layer 12, or the electrons applied to the n-type layer 12 may be stored in the n-type layer 12 and read into the n-type layer 13.
In the front-face bombardment type device, the insulation layer 14 is charge negatively by the input electrons. The input electrons apt to damage the interface of the insulation layer 14 and the n-type layer 12 and the sensitivity is lowered by a dark current. Further, since not only the charge accumulation area but also the charge transfer area are formed on the surface of the p-type substrate 11, an effective area to accept the input electrons, that is, a numerical aperture is reduced.
On the other hand, in the rear-face bombardment type device, the charging of the insulation layer 14 and the damage of the interface of the n-type layer 12 and the insulation layer 14 can be prevented but a structural defect increases significantly. Namely, in order to attain a sufficient resolution power by the rear surface input type device, it is necessary to make the thickness of the p-type substrate 11 to around 10 .mu.m. This makes the manufacturing of the device and setting it on the electron tube very difficult.
As a result, the prior art device which mounts such a solid-state imaging device in the vacuum tube not only lacks the structural stability but also has insufficient sensitivity and noise characteristics as well as life expectancy and reliability.
Thus, the solid-state imaging device as well as the electron tube type imaging device have the foregoing inherent problems.